1. Field of the Invention
The present invention relates to a method for controlling a phase of moulding an annular fixing structure and to a tyre for vehicle wheels including an annular fixing structure. The invention also relates to equipment for moulding and curing green tyres.
2. Description of the Related Art
In the production cycle of a tyre, following a tyre building process in which the various components of the tyre are made and/or assembled, a moulding and curing process is carried out for the purpose of defining the structure of the tyre according to a desired geometry, normally including a specific tread pattern.
For this purpose, the green tyre (in the present text, this term denotes a tyre which has not yet been moulded and cured) is enclosed in a mould cavity which is formed inside a curing mould and which is shaped according to the geometric configuration of the outer surfaces of the tyre to be produced.
When the curing is complete, the mould is opened for the removal of the tyre.
A tyre generally comprises a carcass structure, in the shape of a toroidal ring, including one or more carcass plies, reinforced with reinforcing cords lying in radial planes (in the case of what are known as radial tyres), in other words in planes containing the axis of rotation of the tyre. The ends of each carcass ply are fixed to at least one metal annular structure, usually known as the bead core, which reinforces the beads, in other words the radially inner extremities of said tyre, which serve to fit the tyre on a corresponding mounting rim. A band of elastomeric material, called the tread band, is placed on the crown of said carcass structure, and a relief pattern for ground contact is formed in this tread band at the end of the curing and moulding stages. A reinforcing structure, usually known as the belt structure, is placed between the carcass structure and the tread band. In the case of tyres for a car, this belt structure usually comprises at least two radially superimposed strips of rubberized fabric provided with reinforcing cords, usually metallic, positioned parallel to each other in each strip and crossing the cords of the adjacent strip, the cords being preferably positioned symmetrically about the equatorial plane of the tyre. Preferably, said belt structure also comprises a third layer of fabric or metal cords placed circumferentially (at 0 degrees), in a radially outer position, at least on the ends of the underlying strips.
Sidewalls of elastomeric material are also applied to the corresponding lateral surfaces of the carcass structure, each extending from one of the lateral edges of the tread band to the position of the corresponding fixing annular structure to the beads.
In the present description and in the following claims, the term “circumferential plane” denotes any plane passing through a radial axis of the tyre and perpendicular to the axis of rotation of the tyre.
The term “bead force” denotes a measurement of the resistance of the bead when inflated, in other words a measurement of the resistance offered by the bead to this type of deformation. A high bead force is desirable to prevent the tyre from rotating on the rim in vehicles with high torque in acceleration and/or braking, which would lead to a loss of performance.
US patent application no. US 2003/0141627, published in the name of the Goodyear Tire & Rubber Company, describes a method and equipment for moulding tyres with a radially extendable moulding ring for the beads. When extended, the bead moulding ring has a radial circumferential surface facing outwards for moulding the beads. The bead moulding rings each comprise a plurality of segments, half of which are first segments which are complementary to second segments and alternate circumferentially with the latter.
European patent application EP 0368546 relates to a method and equipment for bladderless tyre moulding. A bladderless tyre moulding press comprises two plates, namely an upper and a lower plate, and means for moving the plates relative to each other from an open position to a closed moulding position, plate mounting means for moulding the sidewalls of the tyre, and means for moulding the tread. Each of the plate mounting means serves to mould a corresponding bead of the tyre, and the bead moulding means include central means which are movable with respect to the corresponding plates for insertion into the tyre. Each plate includes bead moulding means which, when extended, form a surface for moulding an inner surface of the beads. The plates retain the beads and seal them against the sidewall plates of the mould, thus enabling a pressurized fluid to be introduced into the tyre.
US patent application no. 2008/0128947 in the name of Sumitomo Rubber Industries, Ltd relates to a method of making tyres in which a green tyre is inserted into a mould in which an inner plate and an inflatable bladder are present. During insertion, the diameter of the inner plate is reduced and the inflatable bladder is contracted. Following insertion, the diameter of the inner plate is increased in such a way that the surface of a bead of the green tyre bears on the plate. When the gas is introduced, the inflatable bladder is gradually expanded. The mould is locked in such a way that the pressure inside the bladder increases further. A portion of the peripheral inner surface of the green tyre which is not in contact with the plate is in contact with the inflatable bladder. The green tyre is heated by thermal conduction from the mould and from the inflatable bladder.